It is known that previously, when current or voltage transducers were connected, an analog point-to-point connection of the respective transducer to the associated protection or field device was implemented. For this purpose, each protection or field device contained a corresponding number of current and voltage inputs. The current and voltage inputs were implemented by means of a special terminals. The inputs were sampled simultaneously and synchronously with the program sequence of the corresponding protection or field device at a rate of, e.g. 1 . . . 5 ksamples/s. Current and voltage transformers were used for decoupling the inputs in the protection or field device with respect to DC.
The disadvantageous factors of this method and this arrangement, respectively, are the high rated kilovolt-amperes required for the transducers used since, as a result, these become expensive. For this reason, it has been attempted for a number of years to define a new interface with lower power which allows the transducers to be dimensioned more advantageously. At present, there are attempts to define an interface and a message structure for the digital transmission of transducer data. The draft standards IEC 61850-9-1 and IEC 60044-8 are results of these attempts. Both draft standards use the same message contents but different physical transmission means for transmitting the transducer data. In the draft IEC 60044-8, a synchronous serial interface with 2.5 Mbit/s and Manchester coding is proposed for transmitting the transducer data.
FIG. 1 shows a block diagram from these draft standards, which show the basic connection of a transducer 1 followed by a converter, for example an analog/digital converter 2 with associated power supply, to a digital transmission link 3. These draft standards provide for a precisely defined transmission time for a message with samples. The time between the sampling of the measured analog output value of the respective transducer and the reception of the message with the samples is defined. Secondary converters 4 and 5 are provided for matching purposes.
As can be seen in FIG. 2, it is proposed in the draft standards to combine the measured digital values of the transducers with the aid of a so called merging unit, a data concentrator 10, for a branch in, for example, a transformer substation. The data concentrator 10 outputs a message with the measured digital values at an output 11 to a protection or field device, not shown, and is connected to a clock generator, also not shown, via an auxiliary input 12. In FIG. 2, elements 1 and 2 according to FIG. 1 are in each case designated by PC and elements 4 and 5 are designated by SC.
The basis for a digital transmission of the measured digital value, formed from the measured output values of the transducers, to the data concentrator is an equidistant sampling of the individual measured analog output values of the transducers. It is also assumed that the sampling of the measured output values of the various current and voltage transducers also from different branches of the transformer substation must be synchronized in time. Two approaches to synchronizing the sampling present themselves:
The first approach could be a use of the interpolation method. In this method, the different known time delays between the sampling of the samples of the measured output values, sent in datagrams, and the reception of the datagrams in the data concentrator and the measurable delay between the reception of the various datagrams of the individual transducers are used for allocating a sampling time to each received sample which is accurate to the microsecond. Following this, an interpolation is performed between the individual samples in order to recalculate all received samples to a common sampling time. This situation is illustrated in FIG. 3. This figure is taken from the abovementioned draft standard.
The disadvantageous factor of this method is the high expenditure for the time stamping device for the received datagrams, which has an accuracy of approx. 100 ns, and the device required for interpolating the sampled signals in real time. If an interpolation polynomial (e.g. recursive third-order splines interpolation) is used for the interpolation, an additional interpolation error is caused by the interpolation. The result of the interpolation can no longer be described by means of a linear transfer function, i.e. the algorithm used is nonlinear. As an alternative, adaptive filters can also be used for the interpolation. In this case, the relatively high group delay of the all-pass filters (or low-pass filters with a cut-off frequency which is distinctly higher than the bandwidth to be used), which can be implemented in practice, has a disadvantageous effect. For the tracking of the adaptive filters, furthermore, a device is needed in this case by means of which the filter coefficients to be tracked can be calculated. The LMS algorithm, for example, is suitable for this purpose. To implement this algorithm, either a digital signal processor DSP or a complex ASIC is needed.
The second approach consists in utilizing a synchronization pulse throughout the transformer substation. Since the signals provided by the transducers are usually used by various devices in a transformer substation, it is actually never possible to split a transformer substation into individual sections in which a common sampling clock can be used for all measured transducer output values to be synchronized. It is always the entire transformer substation which must be supplied with a central sampling clock. In this case, all transducers can generate samples which are sampled synchronously to one another. FIG. 4 shows the figure used in the aforementioned draft standard for illustrating the synchronously sampled signals.
Methods which operate with a central clock for synchronizing the sampling of various transducers always present problems for reasons of reliability since, in the event of a failure of the central clock, all transducer signals of the entire transformer substation fail together. Redundancy concepts can only mask this fundamental problem since the clock to be used must always be generated at a central point for synchronizing the sampling. Furthermore, this method in principle requires a bidirectional connection to the individual transducers.
In the draft IEC 61850-9-1, a 100-Mbit Ethernet interface is proposed. The use of Ethernet interfaces basically requires a central sampling clock for synchronizing the sampling since it is not possible to guarantee a constant transmission time of the transducer signals via the Ethernet bus with this transmission method. The method for digitally transmitting transducer signals proposed in this draft standard is thus only another variant of implementation of the second method according to draft standard IEC 60044-8.
Both drafts are tailored for the requirements of control and protection systems. Both drafts are unsuitable for recording transients and for power quality measurements (sampling rates are too low). The sampling rates which can be achieved according to IEC 60044-8 and 61850-9-1 are within the range of 1 . . . 5 ksamples/s. The sampling rates required for transient recording and power quality measurements are within the range of 5 . . . 40 ksamples. These two drafts only describe the interface between switchgear and protection and field device. The acquisition and synchronization of the transducer data to one another remains open in these proposed solutions.
From international patent application WO 01/45232, a method for detecting and digitally transmitting measured analog output values of a number of transducers to a protection or field device is known in which the measured analog output values of each transducer are converted into measured digital values which are transmitted to a data concentrator; in the data concentrator, a message containing the measured digital values of the transducers is formed with a predetermined minimum sampling rate and the message is transmitted to the protection or field device at a sampling rate which is higher by a factor m than the minimum sampling rate, the measured analog output values are converted into the measured digital values and they are transmitted, the factor m being an integer divider of the number n of the filter coefficients of in each case one FIR filter (FIR) in the data concentrator for each transducer, from buffers associated with each transducer data are transferred with the clock pulse of a clock generator of the data concentrator into post-buffers preceding the FIR filters (FIR) and the message is assembled by means of a multiplexer from output buffers following the FIR filters (FIR).
The invention is based on the object of developing the known method described above in such a manner that it allows good results to be achieved in every case.
To achieve this object, according to the invention, a method for detecting and digitally transmitting measured analog output values of a number of transducers to a protection or field device is used, in which the measured analog output values of each transducer are converted into measured digital values at a sampling rate which is higher by a factor than the minimum sampling rate and are transmitted, the factor being an integer divider of the number of the filter coefficients of in each case one FIR filter with filter coefficients of the value 1 in a data concentrator for each transducer, the measured digital values are transmitted as a message to a data concentrator, data being transferred from buffers associated with each transducer with a clock pulse into post-buffers preceding the FIR filters, the clock being formed from the synchronous character of the respective message and the fixed clock of a clock generator, and in the data concentrator, a transmit message with measured digital values of the transducers with reduced sampling rate is formed from output buffers following the FIR filters by means of a multiplexer, and the transmit message is transmitted to the protection or field device.
In an advantageous embodiment of the method according to the invention, a message is generated with the measured digital values by means of a clock-synchronous logic arrangement and is transmitted by in each case one transmitter to in each case one data receiver at the data concentrator.
The invention is also related to an arrangement for detecting and digitally transmitting measured analog output values of a number of transducers to a protection or field device and has the object of improving the known arrangement which can be found in WO 01/45232.
According to the invention, this object is achieved in an arrangement for detecting and digitally transmitting measured analog output values of a number of transducers to a protection or field device, in which each transducer is followed by an analog-digital converter, operating with a sampling rate selected to be higher by a factor than the minimum sampling rate, for forming measured digital values; the factor being an integer divider of the number of the filter coefficients of in each case one FIR filter with filter coefficients of the value 1 in a data concentrator for each transducer; the outputs of the analog-digital converters are connected to the data concentrator and a buffer is associated with each transducer at the input of the data concentrator; post-buffers, the outputs of which are connected to the FIR filters, are connected to the buffers, and a multiplexer is connected to output buffers following the FIR filters; the data concentrator has an output, connected to the protection or field device, for sending off a transmit message with measured digital values of the transducers with reduced sampling rate.
In the arrangement according to the invention, the respective analog-digital converter is advantageously followed by a clock-synchronous logic arrangement for forming a message, and a transmitter.